a) Field of the Invention
This invention relates to a metallic substrate for supporting thereon an exhaust gas cleaning catalyst which is inserted in an exhaust system of an automotive vehicle as cleaning means for exhaust gas from the automotive vehicle.
In particular, the present invention is concerned with a honeycomb body which is a principal element of the metallic substrate, has a honeycomb structure formed of a planar band and a corrugated band made of metal sheets and is employed to support an exhaust gas cleaning catalyst. Notably, the present invention adopts a novel fixing method for the components of the honeycomb body and also for the honeycomb body in the metal-made casing, so that an exhaust gas cleaning metallic substrate excellent in both durability and economy can be provided.
b) Description of the Related Art
Metal-made substrates (hereinafter called the "metallic substrates) for supporting thereon exhaust gas cleaning catalysts of the above-mentioned type (for example, catalyst systems making use of Pt, Rh, Pd and the like) have been mounted on automotive vehicles and the like and have entered the stage of commercial use for their excellent properties compared with conventional ceramic substrates such as cordierite substrates.
In the present field of art, the metal substrates are also called metal supports and may be abbreviated as "MS". In the present invention, this abbreviation will also be used.
The above-described metallic substrates (MS) are each composed of:
(1) a honeycomb body of a honeycomb structure (hereinafter called the "metallic honeycomb body") formed by bringing a planar band and a corrugated band, which are made of heat-resistant metal sheets and have a predetermined width, into a stacked contiguous relationship in view of such desires as supporting a cleaning catalyst in a greater amount per unit volume (in other words, increasing as much as possible the area of effective contact between exhaust gas and the cleaning catalyst per unit volume), reducing the back pressure (the resistance to the discharge of exhaust gas) and further reducing the own weight of the metallic substrate as much as possible; and PA1 (2) a metal-made casing for enclosing and fixedly holding the metallic honeycomb body therein (hereinafter called the "metallic casing"). PA1 (i) among the three metallic honeycomb bodies arranged within the metallic casing, the metallic honeycomb bodies at front and rear positions have each been constructed by fixing the planar and corrugated bands as constituent members of said metallic honeycomb body at at least some of points of contact therebetween and also an outer peripheral wall of the metallic honeycomb body and an inner peripheral wall of the metallic casing at at least some of points of contact therebetween; PA1 (ii) among the three metallic honeycomb bodies arranged within the metallic casing, the metallic honeycomb body at a middle position has been constructed without fixing the planar and corrugated bands as constituent members of said metallic honeycomb body at points of contact therebetween and also an outer peripheral wall of the metallic honeycomb body and an inner peripheral wall of the metallic casing at points of contact therebetween.
An abbreviation (H) is used for the metallic honeycomb body because of its honeycomb structure, while an abbreviation (C) is employed for the metallic casing after the word "casing".
The metallic honeycomb body (H) as a principal element of the above-described metallic substrate (MS) is formed, for example, by bringing a planar band, which is made of a heat-resistant steel sheet of 100 .mu.m or less (preferably 50 .mu.m or less) in thickness, and a corrugated band, which has been obtained by forming a similar steel sheet into a corrugated form, into a stacked contiguous relationship and then rolling them together into a spiral form so that a honeycomb body of a honeycomb structure having a number of network-patterned gas flow passages in an axial direction is obtained.
In the present field of art, a variety of metallic honeycomb bodies (H) are known other than the above-described rolled type, depending on the method for fabricating a metallic honeycomb body (H) from a planar band and a corrugated band. For example, are known those of the stacked type, which are each obtained by stacking both the types of bands in layers. Besides, metallic honeycomb bodies of the radial type, S-shaped type, the type that three huge commas are so united as to make a perfect circle, the X-wrapped (swastika-patterned) type or the like are known from Japanese Patent Application Laid-Open (Kokai) No. SHO 62-273050, Japanese Patent Application Laid-Open (Kokai) No. SHO 62-273051, Japanese Patent Application Laid-Open (Kokai) No. HEI 1-218637, Japanese Language Laid-Open Publication (PCT) No. HEI 3-502660, Japanese Patent Application Laid-Open (Kokai) No. HEI 4-227855 or the like.
A typical conventional metallic substrate is illustrated in FIG. 13. Namely, the conventional metallic substrate indicated by MS' is of the construction that a single metallic honeycomb body H' a principal element, is arranged within a metallic casing C.
To distinguish the conventional metallic substrate MS' and metallic honeycomb body H' from those of the present invention, their abbreviations are accompanied by a prime.
Since the metallic substrate MS' is employed in severe thermal environment, that is, an exhaust gas system, the metallic honeycomb body H' and the metallic casing C are firmly fixed together. This is necessary because the metallic honeycomb body H' and the metallic casing C are heated to high temperatures by heat conducted from hot exhaust gas itself and also by exothermic reactions of exhaust gas induced by a cleaning catalyst and in such a high-temperature atmosphere, the large thermal stress is applied to individual elements, in other words, because they have to withstand such thermal stress. They also have to vibrations and the like during running of an automotive vehicle. In general, they are firmly fixed together by brazing or the like.
On the other hand, the planar band and the corrugated band, which make up the metallic honeycomb body H', are fixed together by one of various methods at points or areas of contact therebetween (for example, at some and predetermined ones of such points or areas of contact inside the honeycomb body).
However, the metallic substrate MS' composed of the conventional metallic honeycomb body H' and metallic casing C' cannot withstand use over a long term. Due to thermal stress produced in the above-described high-temperature atmosphere especially in the axial direction of the metallic honeycomb body H' (in other words, the flowing direction of exhaust gas), separation takes place between an outermost peripheral wall of the metallic honeycomb body H' and an inner peripheral wall of the metallic casing C, so that the durability is significantly impaired.
This led to the need for a change to the belief that it is only necessary is to firmly fix the metallic honeycomb body H' and the metallic casing C together.